Introduction To Unix System Programming

CE 466 Operating System Programming

Introduction to UNIX System Programming

Computer Engineering Department Yarmouk University 9/28/2008

UNIX History Bell Labs Research (Ken Thompson) 1969 1978

Very simple, PDP-11

First edition

Seventh edition

By AT&T 1983

System V

1989

System V.4

“Standard” UNIX

By Berkeley Software Distribution (BSD)

BSD 4.1

1980

BSD 4.3

1987

MINIX

1990

LINUX

1991

UNIX Design Principles • Written mostly in C (little assembly which helped in portability). • Written by programmers for programmers • User interface is simple and replaceable. • Time sharing with simple priority-based CPU scheduling ¾ Supports multiple processes. ¾ A process can easily create a new process

• Demand paging to support memory management

Unix Design Principles • File system is multilevel tree ¾Allow users to create their own subdirectories. ¾A data file is a simple sequence of bytes.

• I/O devices are treated similar to data files. • Was simple and easy to understand ¾Algorithms were selected for simplicity ¾Small set of powerful facilities → system easily extended

Abstract View of UNIX Architecture Text editor

Command-line interpreter (shell)

VI

sh

pico

fork( )

a.out

write( )

open( ), read( ), close( ) hardware

who

UNIX kernel

System calls

cc Application programs

C compiler

Unix Interface • UNIX consists of kernel and system programs ¾Kernel provides the file system, CPU scheduling, and memory management through system calls. ¾System programs provide useful functions using kernel-supported system calls. ƒ e.g. compilation, file manipulation, …

• System calls define programmer interface to UNIX . • The set of system programs define the user interface.

System Calls • The set of extended instructions provided by the operating system and defines the interface it provides to user programs. • A system call is a request to the operating system for service that causes the normal CPU cycle to be interrupted and control to be given to the operating system. • They communicate with the OS to request its services ¾ Create, delete and use various "software objects " e.g. processes, files,…, managed by the OS

System Call Groups in UNIX

In Windows

System calls and C library functions Subject

Win32

UNIX

C library

Console I/O

ReadConsole

read

getc, scanf, gets

Console I/O

WriteConsole

write

putc, printf, puts

Comments

Directory Mgt

CreateDirectory

mkdir

N/A

Make a new directory

Directory Mgt

FindClose

closedir

N/A

Close a directory search handle

fclose

CloseHandle is not limited to files

File System

CloseHandle (file handle)

close

File System

CopyFile

open; read; write; close fopen; fread; fwrite; fclose Duplicate a file

Memory Mgt

HeapAlloc

sbrk, or C library

malloc, calloc

Memory Mgt

HeapFree

use C library

free

shmctl

N/A

Shared Memory CloseHandle (map handle)

Shared Memory CreateFileMapping, OpenFileMappingshmget

N/A

Process Mgt

CreateProcess

fork () then execl ()

N/A

Process Mgt

GetCurrentProcess

getpid

N/A

Process Mgt

GetCurrentProcessId

getpid

N/A

IPC

CreatePipe

pipe

popen

IPC

DuplicateHandle

dup or dup2 or fcntl

N/A

System Calls

There are 6 execxx functions

Or use file names CONIN$, CONOUT$

Layered Structure of UNIX User programs Shells, compilers, interpreters, libraries

kernel

System-call interface to kernel Signals terminal handling character I/O system terminal drivers

File system CPU scheduling Swapping Page replacement block I/O system Virtual memory Disk drivers

Kernel interface to the hardware Terminal controllers terminals

Device controllers disks

Memory controllers Physical memory

UNIX Modes of Operation • User: A number of operations (privileged operations) are forbidden • System (Kernel): All operations are allowed including privileged operations. ¾Privileged operations such as: ƒ Interrupt enabling and disabling ƒ Direct access to I/O devices

System vs. Library Calls • System calls run in kernel mode • Library calls run in user mode Libraries: sin(), cos(), atoi(),…

User program

System call interface System call procedures: write(), read(), fork(), . . .

System vs. Library Calls Library function call Interface to library functions Code of a function

User Program

Without trap to the kernel

Interface to kernel

System call Library functions With a trap to the kernel

Kernel

Codes for other system calls

Hardware

Main code of a system call

Example: getpid System Call

General Form of a System Call [Return Value] = System_Call_Name(params) non-negative

, if OK

-1

, not OK

Return Value =

• In case of error, an error code is placed in an external variable errno.

Handling a System Call

#include <stdio.h> extern int errno; void perror(const char *message);

Actions After a System Call

Let’s Put Things Together

Not checking may cause failure for no apparent cause!

UNIX On-Line Man Pages Section 1 2 3 4 5 6 7 8

Contents User commands (Shell commands) OS services (system calls) Library functions Devices, networks, interfaces System file formats Demo programs Miscellaneous (ASCII, etc ) System maintenance

Check the Man Pages (very helpful) • Each section has an introduction %man 2 intro %man 2 fork %man 3 sin

Exercise 1. Print all: ¾ Command line arguments ¾ Environment strings

2. Use the following system call to implement an " ls" command: int system(const char *string);

#include <sys/types.h> #include <sys/stat.h> #include #include <stdio.h> #include <errno.h> #define BUF_SIZE 256 int main (int argc, char *argv []) { int input_fd, output_fd; ssize_t bytes_in, bytes_out; char rec [BUF_SIZE]; if (argc != 3) { printf ("Usage: cp file1 file2\n"); return 1; } input_fd = open (argv [1], O_RDONLY); if (input_fd == -1) { perror (argv [1]); return 2; } output_fd = open (argv [2], O_WRONLY | O_CREAT, 0666); if (output_fd == -1) { perror (argv [2]); return 3; } /* Process the input file a record at a time. */ while ((bytes_in = read (input_fd, &rec, BUF_SIZE)) > 0) { bytes_out = write (output_fd, &rec, bytes_in); if (bytes_out != bytes_in) { perror ("Fatal write error."); return 4; } } close (input_fd); close (output_fd); return 0; }